1948 through 1960 Ford F-1 and F-100 Rear Axle (differential) Swaps

The following was contributed by the 1948 – 1960 Forum users as a group effort.

After "What IFS will fit my truck?", "What late model rear rear end can I put into my 1948 – 1960 Ford truck ?" seems to be the second most asked question by new participants of the 1948 – 1960 forum. We, the regulars (or old farts) have crafted this article to attempt to answer that question and provide information on all the secondary questions associated with your decision. Credit for the information in this article goes to– Various other web sites containing information on the 9" rear end.– Various regular contributors to this forum who shall remain nameless to avoid people hounding thempersonally for more details or autographs.

Ford used various rear differentials in their trucks from 1948 through 1960 (All F-1/F-100s used the 5 X 5 ï¿½ " bolt pattern)

They were all pretty strong and did a good job. However, finding highway gears for the pre-9" rear ends can be difficult and expensive. Additionally, the older, narrow brakes can leave a bit to be desired.

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Ford 8.8" and 9" Background Information

If you’re not building a power house then you can/should give serious consideration to the 8.8" rear end

used in many late model Fords.

It is a good rear end and can handle a fair amount of abuse. (Because the 8.8" axle is adequate for many applications and because there are many more considerations other than width in making your decision as to what rear end to go with, there will be a little 8.8" info in this article) However, the Ford 9" rear end has long been acknowledged by auto enthusiasts of all types (that means the Ch*vy boys too) as the strongest production axle assembly used to date. It comes in various widths and axle ratios and with various mounting systems (though mounted under leaf springs is by far the most common).

What may not be generally understood however, is that not all 9"s are created equal –

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Types of 9" Housings

For one thing, housings come with various brake shoe widths 1 ï¿½", 2.00" and 2 ï¿½ " as well as different spline counts and bearing sizes.

’67-73 Mustang/Cougar – are light duty, with the thinnest housing material, and a 3" diameter housing at U-bolts. They include small axle bearings, 28 and 31 splines.

’69-77 Galaxies (coils), Lincolns (coils), and late pickups (leaf)- are 3 ï¿½ inch in diameter all the way to the backing plate, coil housings have upper control arm mount.

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How To Recognize 9" Housing Centers

They do NOT have a removable rear inspection cover.

’57 – no dimples, flat center band up the center of the rear cover, bottom drain plug.

’58-59 – two dimples on back of housing, flat center band, some had drain holes.

’60-67 have two dimples, flat center band, oil level hole in back cover.

’63-77 Lincoln, LTD, and Thunderbirds had 9 3/8 inch centers, housings were cut away at the gasket surface for ring gear clearance, one curved rib at the front top portion of differential, strong but poor selection of axle ratios.

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Types of 9" Axles

28 spline axles cannot be shortened and re-splined (they’re tapered). ’72 and earlier 31 spline axles have the ability to be shortened .

’73 and later 9" (cars) have a 5 X 5 ï¿½ " bolt circle and the axles cannot be shortened .

’67-73 Mustang axles are identified by wheel flange center, with oval hole/recess – 28 splines, with two large holes/recesses and a counter sunk center – 31 splines Given a choice of 28 or 31 spine axles, you want the stronger 31spline versions.

A 9" complete rear axle is approximately 35 pounds heavier than an 8.8" rear axle with approximately the same components.

It is common to find a 9" in a old Falcon or Comet that has had a HIPO SMALL block with 31 splines and a locker.

If the case has two vertical ribs from the top to middle of the case it is a good IRON type. If, in the very center of this case, there is the letter "N" then this is a NODULAR CASE. This is the strongest factory case made by Ford.

If you’re staying with the stock front axle, you’ll likely want the 5 X 5 ï¿½ " bolt pattern so your wheels will fit all four corners and you only need to carry one spare.

If you’re going with latter model IFS, then chances are you want the rear axle to have the 5 X 4 ï¿½" bolt pattern. The "Volare", "Cordoba", Mustang II and Dakota all use the 5 X 4 ï¿½" bolt pattern. However, the Mopar front hubs have a larger hub diameter that the center hole in the wheel fits over versus the Ford car hub, so while Mopar and Ford both have a 5 X 4 ï¿½" bolt pattern, using one of the aforementioned Mopar based IFS systems requires you to use wheels intended for the Mopar application front and rear. (The Mopar wheels fit the Ford 5 X 4 ï¿½" rear ends just fine).

If, due to availability in your area or cost, you want to use a rear end with the 5 X 5 ï¿½ " bolt pattern but have it changed to a 5 X 4 ï¿½" bolt pattern, there are two basic ways of doing this:

1. Have your 5 X 5 ï¿½ " axles re-drilled by a machine shop to the 5 X 4 ï¿½" bolt pattern. Figure $70 to $100 for this operation, but the quality of work can vary depending upon your machinist’s experience with this operation.

It has been mentioned on another website (and repeated here) that the ’77-’79 T-bird, Cougar, LTD II, Ranchero, and ’73-up Torinos left side (driver’s side) axle happened to be nearly the exact same length as both axles in the ’57-’72 F-100 9" rear end. It has been asserted that two driver’s side axles from the aforementioned passenger cars could be swapped into the truck housing to give a 5 X 4 ï¿½" lug pattern to match all the popular Mopar, MII, and LTD front IFS swaps for our trucks without getting our old axles re-drilled or the use of dangerous adapters.

Given the reputation we have to uphold here as the premier website for all things Ford truck related or maybe we just had some time on our hands…anyway, one of our members decided to prove/disprove this piece of folklore.

Based upon him actually removing axles from one of the above listed cars and from a 57 – 72 F-100 9" and actually trying to do the swap

The left side only axle shaft and either side brake drum out of a ’73-up Torino or ’77-’79 T-Bird, Cougar, LTD II, or Ranchero will install easily in a ’72 Ford F-100 9" axle housing on either side assuming that you use the truck housing’s brake backing plates and brake assemblies, axle retainer plates, and bearing retainer collars. This swap nets the common 5 X 4 ï¿½" lug pattern with the truck housing.

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Caveats

1. The pressed-on bearing retainer ring on the car axle is thicker than the truck retainer.

2. The truck axle retainer plate uses larger bolts (7/16" ?) to secure it through the backing plate to theaxle housing. The car uses smaller 5/16" bolts. The bolt pattern (retainer plate bolt pattern) is also different. The simple solution here is to use the truck retainer on the Cougar axle. The bearing and bearing retainer ring must be removed to do this, but after 25 years, new wheel bearings aren’t a bad idea.

3. The use of the car’s backing plates and brake drums to maintain proper brake offset and parts compatibility was also debated. As mentioned above, the car’s axle retainers and backing plates are drilled for smaller bolts and in a slightly tighter bolt pattern. This precludes the use of the car’s backing plates on the truck housing without reworking the bolt holes and pattern. Since the whole point of this exercise is to find an option for those of us who are too lazy to have our axles re-drilled and would rather put something together with standard parts, this rework isn’t likely to happen. So the car backing plates can be used provided you are willing to modify them enough to bolt them on. If you do use them, get the backing plates from both sides of the donor car so that you get your emergency brake mounting holes. The truck backing plates can be retained to avoid all that though.

4. With the axle installed in the housing, the first logical thing to try was putting the Cougar’s drum on. It fits like it was built that way. The offset spacing and clearance to the truck’s backing plate is perfect. I even measured to make sure that the shoes would contact the drum in the correct place. The shoes center in the drum exactly.

5. It must be stressed, however, Ford used three different widths of brake shoes in these rear ends 1", 2" and 2". Make sure you get car drums, axles, etc. from a donor with the same width of shoes as the truck housing you’re using. (The FTE member research was done using 2" brake examples. Logically, if both donor and recipient vehicles have 1" brakes then they should swap fine as well and if both vehicles have 2" brakes………)

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Ease of installation

Hands down, the easiest rear end to install into one of these trucks is the 57 – 72 F-100 rear end. If you’re installing it in the stock (below the springs) location, it is a bolt in. The spring perches are in exactly the right spot. You need only use larger U-bolts as the older rear ends had a 2.5" diameter housing (in the area of the U-bolts) and the 9" is a 3" diameter.

The (possible) downside is that you get the 5 X 5 ï¿½ " bolt pattern. Great if you’re staying with the stock front end. Not so great if you’re going with latter model IFS. Even your correct pinion angle is preserved with this alternative. It just doesn’t get any easier than this.

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Brakes (i.e. disc or drum) –

Many folks want rear discs. While the improvement to overall braking capability of the vehicle is negligible as your front brakes do approx. 70% of the total braking, there is the "cool factor" or bragging rights of having disc brakes on all four corners. If you’ve got to have rear discs and don’t want to spend about $700. USD for a bolt on kit, then consider –

2. Volvos – a cheap and "secret" 8.8 source is rear wheel drive Volvos from roughly 77-96. Most ratios are in the 3.5 to 4.1 range, lockers and rear discs are fairly common. Early patterns are metric, after 87 they are Ford SAE. Widths vary a bit depending on brakes but most I’ve used (not in trucks though) are about 54" between axle tube ends (that should provide an axle flange to axle flange width of around 58 – 60 inches). Easy to remove the coil spring perches and addleaves. Many retain the very rugged and visually appealing trailing arms. (Also, Volvo is now owned by Ford, so you can say you’re staying true blue.)

3. Late 70’s and early 80’s Lincolns- Many folks have made the Lincoln Versailles and Lincoln Mark IV rear ends (with discs) work quite well in F-1/F-100s.

Special note on Lincoln disc brake rear ends

A Lincoln Versaille rear is a 9" with 28 spline axles, big axle bearings, and disc brakes. Original ratios are 2.47, 2.50, 2.75 or 3.0 with the 2.50 being the most common. Any Ford 9" carrier will fit, as long as it has a fill plug on the side. The original carrier is normally equipped with a CV joint flange, which can be replaced with a U joint yoke. Axle flange to axle flange they are 57 ï¿½" to 58 ï¿½" wide.

If you get one, make sure it’s complete. Apparently, there are virtually no parts available for the disc brakes. Calipers can be rebuilt, but there are not many cores available, no rotors are available new. Pads are available.

User Note on the disc brake rear end problem. I bought rebuilt calipers for mine and I had the new rotors as they came with the rear when I purchased it. These parts were available at my local parts store. They were expensive compared to other units. Speedway and others sell an inexpensive bracket that you weld to the axle in place of the factory caliper bracket. This bracket allows you to use Chevrolet calipers, which are much more affordable and do not have the goofy emergency brake mechanism as part of the caliper.

Give some serious consideration to these points before go this route. (I.E. especially when you can’t get new rotors without having to go the "weld a new bracket to adapt the Che*vy caliper" route).

Wouldn’t it be simpler to use the Explorer disc brake rear end if you’ve got to have rear discs?

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Methods for lowering ride of F-1/F-100s

Now that you’ve selected your new rear end, you probably want to fool with the ride height while you’re installing it. The stock axle housing is mounted below the springs. Probably has 8 leaves with a total thickness of 2 ï¿½" and the spring perch height is approximately 1 ".

Here are various avenues for lowering the rear suspension that we’re aware of:

1. Remove every second leaf from the rear springs (removing a total of 3 leaves). According to Mid-Fifty, keep #1,3,5,7and the main leaf. In theory this will lower the rear end approx. 1.0" between the actual thickness of the three leaves removed and the slightly softer ride due to the 3 leaves being removed. Cost is near zero.

This may not alter the pinion angle. DROP = ï¿½ " – 1" inches

2. Utilize 2" lowering shackles on the rear mounting end of the rear springs. It is recommended that this be combined with moving the front spring shackle up approx. 1 ï¿½" (necessitates drilling new mounting holes in the frame). This latter step will help to avoid altering the pinion angle. The shackles run approx. $90.00 USD a pair circa 2002.

This may result in the upper edge of the rear shackle coming into conflict with the bottom of the bed floor & may impact pinion angle.

DROP = 1 – 2 inches

3. Combine alternatives # 1 & 2 above for approx. 2 to 3" drop .

4. Utilize mono-leaf rear springs. Available from several sources including John’s F-Fun Hundreds & Vintage Ford. This should yield a drop of 2 – 3".These run approx. $350.00 USD a pair circa 2002.

Mono leaf springs are available with regular eyes for 2-3 inches of drop and reversed eyes for an additional 1 inch or approximately 3 to 5 inches total. If using the reversed eye mono-leaves, it is recommended you flip the front shackle mount upside down to avoid the spring binding on the shackle mount.

5. Another alternative is to mount the differential housing on top of the rear springs rather than its stock location under the springs. Cost is a bit of welding to move the spring perches from the top of the axle housing to the bottom and a kit with new perches, U-bolts and pad(about $30.). This is a very simple job.

DROP = 4 – 4 ï¿½ inches

6. Combining #5 above with reversed eye main leaves would provide further drop but now you’re getting into the area of ground scraping.

DROP = approximately 6 inches

It is generally believed that a minimum of 3" travel is required to provide safe and comfortable handling in everyday driving situations. Any of the above combinations that reduce the suspension travel (distance between the frame and the axle housing) to 3" or less should be accompanied with C-sectioning the frame. There should be a rubber bumper mounted to the frame to prevent metal-to-metal (axle housing to frame) contact. This bumper can be cut down, but should not be eliminated.

The March 2002 issue of Street Rodder has an article showing No-Limits Engineering doing it in one manner. Apparently they also offer a kit. Don’t know the price or whether it bears any resemblance to the manner used in the magazine article. Hopefully not…requires extensive welding.

RB’s Obsolete Auto also offer a "kit" that can be adapted to the frame with some welding and cutting. Doesn’t say in their catalog, but it looks like it would give back about 1 ï¿½" of travel. Cost is approx. $110.00 USD . RB’s alternative is the more traditional approach and looks good (cosmetically).

Looking at a picture of either alternative will quickly reveal that C sections aren’t brain surgery. It wouldn’t be that difficult to make your own. But unless you have easy/free access to steel, it would be hard to beat RB’s price. "

Regardless of which alternative or combination of alternatives you use to lower your truck, be sure to check the pinion angle before everything is bolted and welded into place and set it correctly.

A note about pinion angle: Although most rodders already know this, it bears repeating – It is important that the angle between the driveshaft and the pinion gear in the rear end be the same as the angle between the driveshaft and the output shaft of the transmission. If the angles are not the same, you will feel a vibration in the drivetrain while driving.

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Closing Remarks

This article is based primarily upon information provided by many of the regulars in this forum…based upon their experiences and their knowledge as well as some information gleaned from other sources.

More detail pertaining to the swapping of axles from certain cars into the 57 – 60 truck differential housing in another thread started by user Pcmenten (see link below). We expect the discussion to continue, so be sure to revisit that forum for updates.